In general, as a size of a ship is increased, a sudden course change or stop thereof becomes more difficult. Accordingly, a technique of avoiding collision between ships has been proposed. For example, a ship includes means for acquiring information of another ship, such as an Automatic Identification System (AIS) or a radar. Calculation of a risk of collision is executed by using such information of another ship. For example, a method that uses Time-To-Collision (TTC) has been known as a method for calculating a risk of collision between ships. In TTC, expected courses in a case where respective ships maintain speeds and directions thereof at a point of time of expectation are obtained, and a period of time until a point of time when the expected courses intersect with one another is calculated.
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However, in a case where expected course straight lines of two ships as targets for calculating a risk of collision therebetween do not intersect with one another, it may be impossible to calculate TTC. Accordingly, in a related method for calculating a risk of collision between ships by using TTC, it may be impossible to calculate a risk of collision despite a possibility of the risk of collision. For example, although a certain degree of risk exists even in a case where two opposing ships pass one another, it may be impossible to calculate a risk of collision in a case where expected course straight lines thereof do not intersect with one another.